Add default plot types

[kitizz]

Include some commonly used plot types instead of having to define QML every time. Something like "Single", "TripleColumn".

Sub issues:

• [ ] To work well, this will also need a feature to be able to auto create a new line plot for example if a plot handle does not exist
• [ ] And to make autocreation appeasing, themes should be able to be defined, so that the new plots will associate themselves with a certain look and feel.

Question: should the theming be server-side or client-side? Server-side mean easier development of bindings. Client-side means the actual core API is really basic/simple/extendable...